Liquid crystal display device comprising a capacitance insulating film with a refractive index of 1.7 to 2.0 with respect to a light having a wavelength of 632.8 nm

ABSTRACT

There is provided a high-definition liquid crystal display device that can prevent flicker due to a reduction in the pixel potential in a low-frequency drive of about 10 Hz to reduce power consumption. The pixel has a TFT formed of Poly-Si as a switching element. In the pixel, a capacitance insulating film is formed on a planar first electrode on which a comb-shaped second electrode is fanned. When the film thickness of the insulating film is d and the dielectric constant at 10 Hz frequency is ∈, it is given that ∈d≥5×10 −6  m at 10 Hz frequency. The capacitance insulating film does not have a hysteresis characteristic. The refractive index of the capacitance insulating film with respect to a light of a wavelength of 632.8 nm is 1.7 to 2.0.

CLAIM OF PRIORITY

This application is a continuation of U.S. application Ser. No.14/811,904, filed on Jul. 29, 2015. Further, this application claimspriority from Japanese Patent Application No. 2014-153954 filed on Jul.29, 2014, the contents of which are hereby incorporated by referenceinto this application.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to a display device, and moreparticularly, to a liquid crystal display device capable of suppressinga flicker in a low frequency drive.

(2) Description of the Related Art

In a liquid crystal display device, pixels each having a pixelelectrode, a thin film transistor (TFT) and the like are arranged in amatrix form in a TFT substrate. Further, a counter substrate is placedopposite the TFT substrate with a liquid crystal interposed between theTNT substrate and the counter substrate. Then, an image is formed bycontrolling the transmittance of light by liquid crystal molecules foreach pixel.

The viewing angle characteristic is a problem for the liquid crystaldisplay device. The viewing angle characteristic is a phenomenon of thechange in the brightness or chromaticity between when the screen isviewed from the front, and when it is viewed in an oblique direction.The viewing angle characteristic is excellent the In Plane Switching(IPS) mode for driving liquid crystal molecules by an electric field inthe horizontal direction. There are several types of IPS modes. One ofsuch IPS modes is the so-called Fringe Field Switching (FFS) in which,for example, a common electrode is formed in a matted manner on which acomb-shaped pixel electrode is formed with an insulating film betweenthem, to rotate liquid crystal molecules by the electric field generatedbetween the pixel electrode and the common electrode. The FFS mode canachieve a relatively high transmission and is now the mainstream.

A high definition screen is demanded for medium and small sized liquidcrystal display devices, in which the area of each pixel is small. Whenthe pixel area is reduced, the value of the additional capacitance maynot be enough to stabilize the potential of the pixel electrode. PatentDocument 1 (Japanese Patent Application Laid-Open No. 2008-26430)describes a configuration of an FFS type liquid crystal display devicein which metal oxide particles with a high relative dielectric constant,such as BaTiO₃, are dispersed in an application-type transparentinsulating film as an insulating film between a pixel electrode and acommon electrode, in order to increase the capacitance while increasingthe insulating property of the insulating film.

SUMMARY OF THE INVENTION

In liquid crystal display devices for mobile terminal applications suchas smartphones, it is necessary to reduce the circuit power consumption.Low frequency drive and intermittent drive or the like have beenproposed as one method of addressing this problem. The low frequencydrive is a method of reducing the circuit power by reducing the drivefrequency of the liquid crystal display device, for example, to half orone fourth with respect to the standard condition. Further, theintermittent drive is a method of reducing the circuit power byproviding a circuit stop period of several display periods after thewriting of one display period of the liquid crystal display device isperformed.

Meanwhile, a higher screen definition is also required for such adisplay device, in which the screen may have a high pixel density of 440pixels per inch (ppi) or more. In the high definition screen, the areaof the pixel is reduced, and as a result, the capacitance of the pixelis not enough to stably hold the pixel electrode. The potential of thepixel electrode is switched in a short time in a high frequency drive.However, it is necessary to hold the potential of the pixel electrodefor a long time in a low frequency drive or intermittent drive or othermodes, so that a reduction in the potential of the pixel electrodeoccurs in the holding time. Such a change in the potential of the pixelelectrode appears as a flicker, resulting in a degradation of the imagequality.

The problem to be solved by the present invention is to suppress thechange in the pixel electrode potential at a low frequency orintermittent drive in a high definition screen, to prevent the flickerfrom occurring.

The present invention has been made to overcome the above problem. Someof the major aspects are as follows.

(1) There is provided a liquid crystal display device including: a TFTsubstrate in which pixels are formed between scanning lines extending ina first direction and arranged in a second direction, and video signallines extending in the second direction and arrange in the firstdirection; a counter substrate; and a liquid crystal interposed betweenthe TFT substrate and a counter substrate. The pixels are formed at adensity of 400 pixels per inch (ppi) or more in the first direction. Thepixel has a TFT formed of poly-Si as a switching element. In the pixel,a capacitance insulating film is formed in a planar shape on a firstelectrode, and a comb-shaped second electrode is formed on thecapacitance insulating film. When the film thickness of the capacitanceinsulating film is d and the dielectric constant at a frequency of 10 Hzis ∈ in an operating environment of 50 degrees Celsius, it is given that∈d≥5×10⁻⁶ m at 10 Hz frequency. The capacitance insulating film does nothave a hysteresis characteristic. The refractive index of thecapacitance insulating film with respect to a light of a wavelength of632.8 nm is in the range from 1.7 to 2.0.

(2) In the liquid crystal display device described in (1), the firstelectrode is a common electrode and the second electrode is a pixelelectrode.

(3) In the liquid crystal display device described in (1), the cycle inwhich a video signal is written in the first electrode or in the secondelectrode is 10 Hz or less.

(4) In the liquid crystal display device described in (1), when theperiod of rewriting the video signal to the pixel is one frame, theframe is configured with a scanning period and a break period. The cycleof the frame is 10 Hz or less.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic plan view of a liquid crystal display device;

FIG. 2 is a timing chart showing intermittent drive;

FIG. 3 is a cross-sectional view of a portion of a pixel of the liquidcrystal display device;

FIG. 4 is a plan view of a portion of a pixel of a TFT substrate;

FIG. 5 is a cross-sectional view of a portion of a storage capacitance;

FIG. 6 is a graph that plots a dielectric constant required for thecapacitance insulating film in the storage capacitance with respect tothe writing frequency to a video signal line;

FIG. 7 is an example of the hysteresis of the conductor;

FIG. 8 is a graph showing the relationship between the leakage currentthe pixel potential reduction;

FIG. 9 is a configuration of a TFT taking into account the evaluation ofthe leakage current of FIG. 8;

FIG. 10 shows the characteristics of the refractive index to thewavelength of hafnium oxide; and

FIG. 11 is a cross-sectional view of a portion of the storagecapacitance when e capacitance insulating film has a two-layerstructure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be described in detail below by means ofpreferred embodiments.

First Embodiment

FIG. 1 is a schematic plan view of a liquid crystal display device. Asshown in FIG. 1, the liquid crystal display device includes a liquidcrystal display panel PNL including a display unit with display pixelsPX arranged in a matrix of m lines×n columns(where m and n are positiveintegers). The liquid crystal display device also includes a backlightBLT as an illuminating means for illuminating the liquid crystal displaypanel from the back side.

In FIG. 1, scanning lines 10 extending in the horizontal direction arearranged in the vertical direction and video signal lines 20 extendingin the vertical direction are arranged in the horizontal direction. Thescanning lines 10 are denoted by symbols GL1, GL2, and so on from upwardto downward direction in the figure. Then, the video signal lines 20 aredenoted by symbols SL1, SL2, and so on from the left side of the figure.

The pixel PX is formed in a region surrounded by the scanning lines 10and the video signal lines 20. The pixel PX includes the TFT forswitching between a pixel electrode and the video signal line 20, and acapacitance of the pixel CS between the pixel electrode and a commonelectrode COM. The pixel capacitance configured with the capacitanceformed by liquid crystal, as well as the storage capacitance formed byan insulating film between the pixel electrode and the common electrode.The pixel capacitance is dominated by the storage capacitance, so thatthe pixel electrode and the storage capacitance hereinafter may be usedwith the same meaning. A video signal SIG is written in each pixel by asource driver SD mounted in the liquid crystal display panel, through acontrol circuit CTL. A video signal VS is written in each pixelelectrode for each line according to the selected scanning line 10.

Mobile products are battery driven, so that it is necessary to suppresspower consumption. The power consumption can be suppressed by using alow frequency drive. However, when the low frequency drive is simplyused, it is difficult to display a natural image for video display. Thisproblem has been addressed by the use of a so-called intermittent drivedesigned to drive with a normal frequency for video display whilereducing the frequency of writing to each pixel for still image display.

FIG. 2 is a timing chart for such an intermittent drive. In theintermittent drive, one frame period is configured with a scanningperiod and a following break period. In the scanning period, thescanning lines GL1, GL2, and so on to GLm are selected in series, sothat the TFTs, each of which is the pixel switch of the correspondingline, are sequentially brought into conduction. Then, the video signalVS output to the signal line 20 from the source driver is written andheld in the pixel electrode of each line according to the timing atwhich the pixel switch of each line is electrically conductive. Thereare n video signal lines in the display area, but to simplify thedescription, it focuses on only one signal line, in which VS is thevideo signal corresponding to the particular signal line.

In the break period, any of the scanning lines GL is not selected, andthe video signal held in each pixel electrode is continued to be held.Although the same operation is performed also in the next frame period,the polarity of the video signal is reversed for every frame, so thatthe video signal held by the pixel electrode is reversed for everyframe. As a result, the potential applied to the pixel electrode has arectangular waveform as shown in V (D1), V (D2) and so on to V (Dm).

Also in the intermittent drive, similar to the case of the low frequencydrive, the pixel electrode is required to hold the same potential for along time. However, the pixel capacitance is reduced as the area of thepixel becomes small, so that it is difficult to hold the potential. Inorder to hold the pixel potential, reduction in the leakage current ofthe TFT can be effective. However, in the intermittent drive, thewriting time should be reduced to support video display, so that it isdifficult to use a-Si with small leakage current for he TFT. Thus, it isnecessary to use poly-Si with a large mobility and with a large leakagecurrent.

FIG. 3 is a cross-sectional view of a pixel portion of the liquidcrystal display panel. In FIG. 3, a first base film 101 of SiN as wellas a second base film 102 of SiO₂ are formed by chemical vapordeposition (CVD) on a glass substrate 100. The role of the first basefilm 101 and the second base film 102 is to prevent a semiconductorlayer 103 from being contaminated by impurities from the glass substrate100.

The semiconductor layer 103 is formed on the second base film 102. Thesemiconductor layer 103 is formed in such a way that an a-Si film isformed by CVD on the second base film 102 and converted into poly-Sifilm by a laser anneal. Then, the poly-Si film is patterned byphotolithography.

A gate insulating film 104 is formed on the semiconductor film 103. Thegate insulating film 104 is SiO₂ film prepared by tetraethoxysilane(TEOS). This film is also formed by CVD. A gate electrode 105 is formedon the gate insulating film 104. As shown in FIG. 4, the scanning line10 also functions as the gate electrode 105. The gate electrode 105 isformed, for example, of MoW film. When there is a need to reduce theresistance of the gate electrode 105 or the scanning line 10, an Alalloy is used.

The gate electrode 105 is patterned by photolithography. In thepatterning process, impurities such as phosphorus and boron are doped inthe poly-Si layer form a source S or a drain D in the poly-Si layer.Further, the photoresist for patterning of the gate electrode 105 isused to form a lightly doped drain (LDD) layer between a channel layerof layer and the source S or drain D.

After that, an interlayer insulating film 106 is formed of SiO₂ so as tocover the gate electrode 105. The role of the interlayer insulating film106 is to isolate the gate wiring 105 and a contact electrode 107. Athrough hole 120 is formed in the interlayer insulating film 106 and thegate insulating film 104 so as to connect the source portion S of thesemiconductor layer 103 to the contact electrode 107. Photolithographyprocesses to form the through hole 120 in the interlayer insulating film106 and in the gate insulating film 104 are performed at the same time.

The contact electrode 107 is formed on the interlayer insulating film106. The contact electrode 107 is connected to the pixel electrode 112through a through hole 130. The drain D of the TFT is connected to thevideo signal line 20 shown in FIG. 4 through a through hole 140.

The contact electrode 107 and the video signal line are formed in thesame layer at the same time. The contact electrode 107 and the videosignal line use, for example, an AlSi alloy to reduce the resistance.The AlSi alloy forms a hillock, and Al diffuses into other layers. Forthis reason, the AlSi alloy has a structure in which AlSi is sandwiched,for example, by a barrier layer of MoW, not shown, and a cap layer.

An inorganic passivation film (insulating film) 108 is formed so as tocover the contact electrode 107 to protect the whole TFT. Similarly tothe first base film 101, the inorganic passivation film 108 is formed byCVD. Note that the inorganic passivation layer may be deleted ifreliability remains high. An organic passivation film 109 is formed soas to cover the inorganic passivation film 108. The organic passivationfilm 109 is formed of photosensitive acrylic resin. The organicpassivation film 109 may also he formed of other materials such assilicon resin, epoxy resin, and polyimide resin, in addition to acrylicresin. The organic passivation film 109 has a role of a flattering filmand is made thick. The film thickness of the organic passivation film109 is 1 to 4 μm and often about 2 μm.

The through hole 130 is formed in the inorganic passivation film 108 andthe organic passivation film 109 in order to establish electricalconductivity between the pixel electrode 110 and the contact electrode107. The organic passivation film 109 uses photosensitive resin. Afterapplication of the photosensitive resin, the resin is exposed. Then,only the illuminated portion is dissolved in a specific developer. Inother words, by using the photosensitive resin, it is possible to omitthe formation of the photoresist pattern. After the through hole 130 isformed in the organic passivation film 109, the organic passivation filmis baked at about 230 degrees Celsius to complete the organicpassivation film 9.

After that, an indium tin oxide (ITO) that serves as the commonelectrode 110 is formed by sputtering. Then, patterning is performed toremove the ITO from the through hole 130 and the periphery thereof. Thecommon electrode 110 can be formed in a planar shape commonly to each ofthe pixels. Then, SiN is formed by CVD over the entire surface to formthe common electrode 110. Then, in the through hole 130, a through holeis formed in the capacitance insulating film 111 and the inorganicpassivation film 108 to establish electrical conductivity between thecontact electrode 107 and the pixel electrode 112. If the inorganicpassivation film is not present, the through hole is formed of a singlelayer of the capacitance insulating film.

After that, ITO is formed by sputtering and patterned to form the pixelelectrode 112. FIG. 4 is a plan view of the pixel electrode 112. Thepixel electrode 112 has a slit 1121. FIG. 4 will be described in detailbelow. An orientation film material is applied onto the pixel electrode112 by flexographic printing, inkjet printing or the like. The alignmenttreatment of the alignment film 113 is performed by rubbing method or aphoto alignment method using polarized ultraviolet light.

When a voltage is applied between the pixel electrode 112 and the commonelectrode 110, lines of electric force are generated as shown in FIG. 3.Liquid crystal molecules 301 are rotated by this electric field tocontrol the amount of light passing through a liquid crystal layer 300for each pixel to form an image. Further, a storage capacitance isformed between the pixel electrode 113 and the common electrode 110 withthe capacitance insulating film 111 between them. The storagecapacitance has the role of holding the potential of the pixelelectrode. The purpose of the present invention is to stably hold thepixel potential by defining the storage capacitance in order to reducethe flicker in the low frequency drive or intermittent drive.

In FIG. 3, the counter substrate 200 is arranged opposite the TFTsubstrate 100 with the liquid crystal layer 300 between them. A colorfilter 201 is formed inside the counter substrate 200. The color filter201 includes red, green, and blue color filters that are formed. in eachof the pixels, and in this way a color image is formed. A black matrix202 is formed between the color filters 201 to improve the contrast ofthe image. Note that the black matrix 202 also has a role of a lightshielding film of the TFT, preventing the light current from flowinginto the TFT.

An overcoat film 203 is formed so as to cover the color filter 201 andthe black matrix 202. The color filter 201 and the black matrix 202 havean uneven surface, so that the surface is smoothed by the overcoat film203. The alignment film 113 is formed on the overcoat film to determinethe initial alignment of the liquid crystal. The alignment treatment ofthe alignment film 113 is performed by rubbing method or a photoalignment method, similarly to the case of the alignment film 113 on theside of the TFT substrate 100.

FIG. 4 is a plan view of a pixel portion. In FIG. 4, the pixel electrode112 is formed in a region surrounded by the scanning limes 10 and thevideo signal lines 20. The pixel electrode 112 in FIG. 4 has arectangular profile with the slits 1121 therein. The region between theslits 1121 is the comb-shaped pixel electrode. When the width of thepixel is reduced, the width of the pixel electrode 112 is also reduced,and the pixel electrode 112 may even have only one tooth. In such acase, there is no slit the pixel electrode.

The TFT is forged between the video signal line 20 and the pixelelectrode 112. The semiconductor layer 103 is connected to the videosignal line 20 through the through hole 140, extending below the videosignal line 20, further passing under the scanning line 10, andextending parallel o the scanning line 10. Then, the semiconductor layer103 passes under the scanning line 10 again and extends to the side ofthe pixel electrode 112. Then, the semiconductor layer 103 is connectedto the contact electrode 107 through the through hole 120. The contactelectrode 107 is connected to the pixel electrode 112 through thethrough hole 130.

The semiconductor layer 103 is doped with impurities, except below thescanning lines 10. The semiconductor layer 103 acts as a conductor. Inthe portion where the semiconductor layer 103 passes under the scanningline 10, a channel of the TFT is formed. Thus, in FIG. 4, the TFT has adouble gate structure in which two channel portions are formed inseries. By using the double gate structure, the leakage current in theTFT is reduced to prevent the change in the potential of the pixelelectrode.

FIG. 5 is a cross-sectional view taken along line A-A of FIG. 1. In FIG.5, the common electrode 110 formed in a planar shape and the comb-shapedpixel electrode 112 face each other with the capacitance insulating film111 between them. When a voltage is applied to the pixel electrode 112,lines of electric force are generated as shown in FIG. 5 between thepixel electrode 112 and the common electrode 110 to drive the liquidcrystal, and at the same time, electric charges are accumulated in thestorage capacitance with the capacitance insulating film 111 betweenthem.

In order to increase the storage capacitance in the limited area of thepixel electrode 112, it is necessary to increase the dielectric constantof the capacitance insulating film. However, the problem is not solvedonly by increasing the dielectric constant. It is to be noted that, inthis specification, the relative dielectric constant is simply referredto as dielectric constant. The first is that the dielectric constant ofan insulator has a frequency characteristic. Thus, the dielectricconstant at the frequency to be used is a problem. In the presentinvention, it is necessary to ensure that the flicker does not occureven when the signal write cycle is 10 Hz, so that the dielectricconstant at 10 Hz is a problem.

Further, the liquid crystal display device may also be used in anenvironment of approximately 50 degrees Celsius, and the dielectricconstant at 50 degrees Celsius is required to hold the necessary valueso that the liquid crystal display device can operate also at this levelof temperature. FIG. 6 is a graph that plots the dielectric constantrequired for the capacitance insulating film 111 to obtain the storagecapacitance that is unlikely to allow the flicker to occur when the filmthickness of the capacitance insulating film 111 is 100 nm, for everywrite frequency both at 25 and 50 degrees Celsius.

As shown in FIG. 6, the required dielectric constant is 49.9 at 50degrees Celsius. In other words, it is possible to prevent the flickereven in the 10 Hz drive as long as the dielectric constant is 50 ormore. On the other hand, at 25 degrees Celsius, the dielectric constantthat can prevent the flicker is 20.3 even in the 10 Hz drive. In otherwords, it is possible to prevent the flicker at 20 degrees Celsius evenin the 10 Hz drive as long as the dielectric constant is 21 or more.

Note that in FIG. 6, the dielectric constant required for thecapacitance insulating film at 50 degrees Celsius is large, because theleakage current of the TFT is increased particularly at a hightemperature. In other words, poly-Si has a large mobility and canincrease the signal write speed, which is advantageous in thehigh-frequency drive, but there is a problem of a large leakage current.Further, a-Si has a small leakage current, whose carrier mobility is onetwentieth or less of that of poly-Si, so that a problem occurs in thehigh frequency writing or when switching to the intermittent drive andto the normal drive.

When the thickness of the capacitance insulating film 111 is 100 nm, adielectric constant of 50 or more is required for the capacitancedielectric constant. This means that when the thickness of thecapacitance insulating film 111 is different, the required dielectricconstant is also different. The capacitance is proportional to thedielectric constant of the insulating film and is inversely proportionalto the film thickness of the insulating film. Thus, if dielectricconstant×film thickness=∈d is given as a parameter, the value of ∈dshould be greater than a predetermined value.

When the operating temperature is 50 degrees Celsius, it is given∈d=50×100×10⁻⁹ m=5×10⁻⁶ m. In other words, at 50 degrees Celsius, it ispossible to suppress the flicker as long as ∈d is 5×10⁻⁶ m or more.Similarly, at 25 degrees Celsius, it is given that ∈d=21×100×10⁻⁹m=2.1×10⁻⁶ m. In other words, at 50 degrees Celsius, it is possible tosuppress the flicker as long as ∈d is 5×10⁻⁶ in or more.

Another important feature of the capacitance insulating film 111 is thatthe capacitance insulating film 111 has no hysteresis characteristic.Barium titanate (BaTiO₃) has a very large dielectric constant simply interms of the increase in the capacitance of the pixel. However, bariumtitanate has a hysteresis characteristic as shown in FIG. 7. In FIG. 7,the horizontal axis represents the electric field E that is applied tothe insulating film, and the vertical axis represents the polarization Pof the insulating film. The liquid crystal display device performs ACdriving as shown in FIG. 2. Thus, a possibility arises that thebrightness changes for each polarity due to the hysteresis. For thisreason, it is necessary that the capacitance insulating film does nothave the hysteresis characteristic such as shown in FIG. 7.

FIG. 8 is a graph showing the relationship between the leakage currentof the TFT and the reduction in the pixel potential at 440 ppi. In FIG.8, the vertical axis represents the change in the pixel potential by mv,and the horizontal axis represents the leakage current by A/cm². Inother words, the flicker is visible when the reduction in the pixelpotential is large in a predetermined signal writing time period. InFIG. 8, when the reduction in the pixel potential is 5 mV or less asindicated by the line NF, the flicker is not visible.

The reduction in the pixel potential is determined by the capacitance ofthe pixel and the leakage current. The capacitance of the pixel seems tobe dominated by the storage capacitance. Thus, it is determined by thedielectric constant of the capacitance insulating film 11. On the otherhand, the leakage current seems to be dominated by the leakage currentof the TFT. FIG. 8 is an evaluation of the relationship between theleakage current and the pixel potential reduction, under suchassumption.

The leakage current is obtained from the evaluation of the leakagecurrent in the configuration of the TFT shown in FIG. 4. FIG. 9 is anenlarged plan view of a only portion of the TFT shown in FIG. 4. Theoperation of the TFT in FIG. 9 is the same as that described in FIG. 4.The TFT in FIG. 9 has a double gate structure in which the channel isformed at two locations. The width of each channel is w, and the lengththereof is d. FIG. 8 shows the evaluation when the channel width w is1.5 μm and 2.5 μm. The greater the channel width the greater the leakagecurrent is. Further, the leakage current is the value when the electricfield strength is 2 MV/cm.

In FIG. 8, when the dielectric capacitance of the capacitance insulatingfilm is 6.5, the potential change of the pixel electrode is as large as5 mV or more. Further, it is also possible to clearly observe theinfluence on the pixel potential change by the channel width of the TFT.On the other hand, when the dielectric constant of the capacitanceinsulating film is 50, it is possible to suppress the flicker until theleakage current is 5×10⁻⁷ A/cm². At this time, the influence of thechannel width of the TFT is also small. This is because the storagecapacitance is originally large, so that the pixel potential is notreduced even with a small amount of leakage current.

As described above, by setting the dielectric constant of thecapacitance insulating film to 50 or more and by using a material withno hysteresis, even if the area of the pixel is small, it is possible tosuppress the flicker even when a low-frequency drive of about 10 Hz orintermittent drive is performed. The present embodiment assumes that thepixel density is 440 ppi. However, in a higher definition display, thepixel area is further reduced, in which the leakage is increased and theflicker is more likely to occur. As a result, the higher definitiondisplay tends to require a larger dielectric constant. On the otherhand, when the drive frequency is set to 20 Hz instead of 10 Hz, therequired dielectric constant can be half the dielectric constant at 10Hz. As a result, both the definition of the product and the desireddrive frequency are taken into account, so that the optimal dielectricfilm is selected.

Note that the capacitance insulating film 111 used for the storagecapacitance includes the common electrode 110 of ITO on the lower side,and the pixel electrode 112 of ITO on the upper side. Thus, when therefractive index of the capacitance insulating film 111 is greatlydifferent from the ITO, the reflection at the interface between thecapacitance insulating film 111 and the ITO increases, and the screenbrightness is reduced. The refractive index is dependent on thewavelength. In general, the evaluation is performed using the refractiveindex of the light of a wavelength of 632.8 nm. In other words, therefractive index of the capacitance insulating film 111 used for thestorage capacitance with respect to a light of a wavelength of 632.8 nmshould be set to a range from 1.7 to 2.0, which is close to therefractive index of the ITO.

Second Embodiment

As described in the first embodiment, by setting the dielectric constantof the capacitance insulating film 111 of the storage capacitance to 50or more, it is possible to suppress the flicker also in an environmentof 50 degrees Celsius in a pixel area of 500 ppi or less. However, whenthe liquid crystal display device is not required to operate in atemperature environment as high as 50 degrees Celsius, or in othercases, the leakage current or the like of the TFT is small, and there isno need to increase the storage capacitance to a very high level. As aresult, the dielectric constant of the capacitance insulating film usedfor the storage capacitance can also be reduced.

As described with reference to FIG. 6, when the film thickness of thecapacitance insulating film 111 is 100 nm, it is possible to increasethe storage capacitance to a level that can prevent the flicker fromoccurring at 25 degrees Celsius, as long as the dielectric of thecapacitance insulating film 111 is 21 or more. Hafnium oxide (HfO₂)whose dielectric constant is 25 at 25 degrees Celsius can meet thiscondition. In the case of hafnium oxide, the dielectric constant is 25,which is greater than 21, so that a visible flicker does not occur evenif the use temperature slightly exceeds 25 degrees Celsius. Further, bysuppressing the leakage current by changing the configuration of the TFTor by other modifications, it is possible to suppress the flicker alsoin a normal environment that is higher than 25 degrees Celsius.

FIG. 10 shows the wavelength dependence of the refractive index ofhafnium oxide. As shown in FIG. 10, the refractive index of hafniumoxide with respect to a light of a wavelength of 632.8 nm is 1.9, whichis close to the refractive index of the ITO and meets the range of therefractive index from 1.7 to 2.0.

Third Embodiment

There is a possibility that the leakage current may increase in theinsulating film whose dielectric constant configuring the storagecapacitance is 50 or more. When the leakage current is large, thereduction in the pixel potential occurs even if the capacitance has beenincreased, and as a result, the flicker will occur. In the presentembodiment, as shown in FIG. 11, the capacitance insulating film 111 hasa two-layer structure in which SiN film 1111 is formed on the lowerlayer. SiN has a high resistivity and can suppress the leakage current.

Meanwhile, in this case, the capacitance is a series connection. Thus,if the SiN film 1111 is made thick, the storage capacitance isdetermined by the SiN film 1111 even if a high dielectric constantinsulating film 1112 is used. As a result, it makes no sense to use thehigh dielectric constant insulating film 1112. Thus, in this case, thefilm thickness of the SiN film is preferably 70 nm or less.

In order to increase the storage capacitance, the capacitance insulatingfilm 111 may be formed by using a single film of the SiN film to reducethe film thickness of the SiN film. However, if the film thickness ofthe SiN insulating film is reduced to an extremely low level, a pinholeoccurs and the common electrode may not be isolated from the pixelelectrode. For this reason, it is difficult to suppress the flicker in alow frequency drive of about 10 Hz by using the SiN film alone.

Meanwhile, as in the case of the present embodiment, the laminateconfiguration of the high dielectric constant insulating film and theSiN film allows to form a storage capacitance with a high capacitance, asmall leakage current, and good isolating properties, in such a way thatthe common electrode and the pixel electrode are kept isolated by usingthe high dielectric constant insulating film or using the highdielectric insulating film and the SiN film, and that the SiN film isallowed to reduce the leakage current.

As for the film thickness in FIG. 11, it is preferable that thethickness d1 of the SiN film 1111 on the lower layer and the thicknessd2 of the high dielectric constant insulating film 1112 on the upperlayer are approximately the same, or the thickness d2 of the upper highdielectric constant insulating film 1112 is made greater than thethickness d1 of the lower SiN film 1111. When the storage capacitance isdivided into a capacitance CS2 on the side of the dielectric constantinsulating film and a capacitance CS1 on the SiN side, the capacitanceof the whole storage capacitance is dominated by the capacitance CS1 onthe SiN side which is smaller than the dielectric constant. For thisreason, increasing the film thickness on the SiN side of the lower layeris not a good idea.

Note that in the above description, the SiN film 1111 is placed on thelower layer and the high dielectric constant insulating film 1112 isplaced on the upper layer. However, the capacitance insulating film 111can also be configured such that the high dielectric constant insulatingfilm 1112 is placed on the lower layer and the SiN film 1111 is placedon the upper layer. In this case also, it is preferable that the filmthickness of the high dielectric constant insulating film 1112 is equalto the film thickness of the SiN film 1111, or that the film thicknessof the high dielectric insulating film 1112 is greater than the filmthickness of the SiN film 1111.

By also configuring the condition of the high dielectric constantinsulating film in this embodiment as described in the first or secondembodiment, it is possible to suppress the occurrence of the flickercompared to the case of using the SiN film alone. Note that by settingthe refractive index of the high dielectric constant insulating film tothe range from 1.7 to 2.0, it is possible to eliminate nearly allreflections on the interface with the SiN film.

The foregoing embodiments have focused on the configuration in which thecommon electrode is formed in a planar shape on the organic passivationfilm, on which the pixel electrode with a slit or a comb-like shape isformed through the capacitance insulating film. However, the presentinvention can also be applied in the same manner to the configuration inwhich the pixel electrode is formed in a planar shape on the organicpassivation film, on which the common electrode with a slit or acomb-like shape is formed through the capacitance insulating film.Further, the present invention is based on the assumption of using thecomb-like electrode shape in the IPS mode. However, the presentinvention can also be applied to the case in which a transparentcapacitance is formed over the entire pixel surface by using a planarITO electrode, a capacitance film, and a planar pixel electrode in theTN and VA modes.

As described above, by applying the present invention, it is possible torealize a liquid crystal display device capable of preventing theflicker even in a low frequency of 10 Hz or intermittent drive. This is,of course, means that is possible to prevent the flicker, or the like,even in the drive range from 10 Hz or more to the normal frequency of 60Hz.

What is claimed is:
 1. A liquid crystal display device comprising: afirst substrate including pixels formed between scanning lines extendingin a first direction and arranged in a second direction, and videosignal lines extending in the second direction and arranged in the firstdirection; a second substrate; and a liquid crystal interposed betweenthe first substrate and the second substrate, wherein the pixel has aTFT as a switching element, wherein in the pixel, a capacitanceinsulating film is formed on a first electrode formed in a planar shapeon which a comb-shaped second electrode formed, wherein the refractiveindex of the capacitance insulating film with respect to a light of awavelength of 632.8 nm is in the range from 1.7 to 2.0.
 2. A liquidcrystal display device according to claim 1, wherein the capacitanceinsulating film does not have a hysteresis characteristic.
 3. A liquidcrystal display device according to claim 1, wherein the first electrodeis a common electrode and the second electrode is a pixel electrode. 4.A liquid crystal display device according to claim 1, wherein the cyclein which a video signal is written in the first electrode or in thesecond electrode is 10 Hz or less.
 5. A liquid crystal display deviceaccording to claim 1, wherein when the period of rewriting the videosignal to the pixel is one frame, the frame is configured with ascanning period and a break period, wherein the cycle of the frame is 10Hz or less.
 6. A liquid crystal display device according to claim 1,wherein when the film thickness of the capacitance insulating film is dand the dielectric constant at a frequency of 10 Hz is ∈ in an operatingenvironment of 50 degrees Celsius, it is given that ∈d≥5×10⁻⁶ m at 10 Hzfrequency.
 7. A liquid crystal display device according to claim 1,wherein when the film thickness of the capacitance insulating film is dand the dielectric constant at a frequency of 10 Hz is ∈ in an operatingenvironment of 25 degrees Celsius, it is given that ∈d≥2.1×10⁻⁶ m at 10Hz frequency.
 8. A liquid crystal display device according to claim 7,wherein the capacitance insulating film is formed of hafnium oxide(HfO₂).
 9. A liquid crystal display device according to claim 1, whereinthe capacitance insulating film is formed by a first layer and a secondlayer, wherein the first layer is formed of SiN and the second layer isformed by a high dielectric constant insulating film, wherein when thefilm thickness of the high dielectric constant insulating film is d andthe dielectric constant at a frequency of 10 Hz is ∈ in an operatingenvironment of 50 degrees Celsius, it is given that ∈d≥2.1×10⁻⁶ m at 10Hz frequency.
 10. A liquid crystal display device according to claim 9,wherein the thickness of the first layer is equal to or less than thethickness of the second layer.
 11. A liquid crystal display deviceaccording to claim 9, wherein the first layer is formed on the lowerside of the second layer.